Flexible packaging, particularly for food, is subject to many demands. The packaging needs to be workable in such a way that the packaging material may be quickly placed around the item to be packaged using machinery. The packaging material must also be of such a quality that it adequately stores the product before the packaging is open. In the case of food products, this typically means that the packaging materials provide an oxygen barrier to maintain freshness.
Ethylene vinyl alcohol (EVOH) is an extrudable resin that has excellent oxygen, flavor, and aroma barrier properties. EVOH resins and packaging materials have been used for several decades as meat and cheese film wrappers and the barrier properties of EVOH with respect to oxygen, grease, oil, flavor additives, and aroma is well understood.
However, when exposed to humidity levels of 85% or higher, the barrier properties of EVOH degrade. To avoid this degradation, the EVOH is typically extruded in a multi-layer symmetrical coextrusion in which specialized tie resins are used to adhere the EVOH to outer polyolefin layers that protect the EVOH from humidity. For example, a three resin, five layer coextrusion of EVOH may include LDPE-Tie Resin-EVOH-Tie Resin-LDPE. In this five layer structure, the LDPE (low density polyethylene) layers protect the EVOH layer from exposure to moisture. Also, the LDPE and Tie Resin are extruded each from one extruder where they are split into two layers and directed to either side of the EVOH layer by a feedblock device. The LDPE and Tie Resin are exactly the same material on both sides of the EVOH, thus it is called a symmetrical coextrusion.
It should be further understood that, when melted, EVOH becomes fluid very quickly with very low melt strength and cannot be extruded as a separate thin film. In extrusion coating, a melt curtain is formed between the exit of the extruder die and the extruder nip. The melt curtain is completely unsupported in this air gap between the die and the extruder nip. Therefore, the melt curtain needs to have enough viscosity and melt strength to keep from tearing apart. Thus, EVOH has traditionally required the above-described five layer structure not only to maintain its barrier properties, but also to provide adjacent structural layers (such as polyethylene (PE)) that physically support the coextruded layer of the EVOH. Thus, while EVOH has excellent oxygen barrier properties, EVOH has been considered unsuitable as a barrier layer in some applications for a number of reasons. For one, production of EVOH has required extrusion machines capable of coextruding more than two resins. Such machines are very expensive and not as common as those extruding only two resins. Further, dual tie resin and polyolefin layers are provided on both sides of the EVOH coextrusion to protect the EVOH layer from humidity; however, these multiple layers can increase the material cost of the packaging and increase the thickness of the packaging. This has made EVOH-based packaging less competitive for certain packaging applications.
Hence, there is a continuing need for improved and lower cost EVOH packaging and a method of making the same.